1. Field of the Invention
The invention relates to a method for drying products which are in a divided form, for example grains, particles or small plates. It also applies advantageously to the drying of products the drying kinetics of which is limited by the internal diffusion of water and which desirably have to be dried in depth, with a drying uniformity from grain to grain, providing that the grains, during drying, do not lose their qualities by thermal decomposition. By way of examples of such products, cereals may be mentioned such as corn and maize and other food seeds such as sunflower seeds. The invention also relates to apparatuses for implementing this method.
This method is based on the principle of placing the product to be dried in suspension in a gaseous stream constituting the hot drying fluid. A thick layer of the product to be dried is formed and a rising stream of said hot fluid is caused to pass therethrough, where it then takes the appearance of a boiling fluid, being continuously stirred, and it occupies the whole of the space which is reserved for it up to its free surface, in the manner of a liquid, the speed of the gaseous stream being adapted to the physical characteristics of the product. This technique, called "fluidized bed drying", allows to improve the drying phenomena at the level of the thick layer, thus promoting thermal exchanges.
As a matter of fact, dihedral duct driers are known for the drying of cereals. Such driers are very often divided into several superposed boxes, where the thermal treatment is different as the grain moves down by gravity around the ducts which distribute the hot air inside the grain mass. However, the air/product contact is not optimum and preferential air passages are formed causing a heterogeneous treatment of the product. Furthermore, in order to be effective, such apparatuses must be of a very large size and require considerable investments; the smaller driers of this type, called "farm driers" have very poor yields (about 1500 kcal/kg of evaporated water), the residence time of the grain in the apparatus varying between 5 and 11 hours.
In the agro-alimentary field which, as this was mentioned above, forms one of the fields of application of the present invention, because the agro-alimentary products often require drying before packing, different types of driers are known using the fluidized bed drying principle, these driers being able to be classed in two types:
the first type is that of driers in which the drying agent must provide for the transport and drying of the product, and in which is included the drier described in the Czech patent n.degree. 183 578, which is a single stage driers, and the drier described in French Patent N.degree. 83 00674 which is a multi-stage drier. In driers of this type, the uniformity of the thermal treatment, provided by putting the grains in suspension, is largely impaired by a bad distribution of the residence times owing to the random displacement of the particles. Furthermore, single stage driers are generally poorly adapted to the drying of particles, when the internal diffusion of the bound water is the limiting step of the process. As a matter of fact, the apparatus must be adapted so that the minimum residence time of the product corresponds to the mean residence time so as to obtain an adequate treatment of the grains. However, the total residence time of the product must be optimized so as to retain the food quality thereof. Because of the large dispersion of the residence times, the size of the apparatuses is relatively large and this results in the use of very high air rates, the imperative recycling of which is difficult to carry out. PA1 the second type is that of driers in which the drying agent provides for drying of the product, whereas the transport thereof is provided by mechanical means. Belonging to this type are the drier disclosed in Canadian Patent n.degree. 160431, which is a single stage drier, and that disclosed in the Bulgarian publication A.I. DRAGANOV: "Trudove Na Nauchnoiezsledovatelskija Institut Po E'rnosakhranenie E'rnoprerabotka I Khlebproproizvodstvo (SOFIA), 1971; V.2; 61-73", which is a four stage drier. In the driers of this second type, the distribution of the residence times is perfectly controlled by mechanical means, which consist of vertical baffles causing displacement of the grain layers over perforated plates which act as gas distributors. The apparatuses of this type having only one stage have the same drawbacks as those mentioned above. Insofar as the Bulgarian apparatus is concerned, its design does not allow the air to pass from one stage to another. Each stage is fed alternately with a hot air (125.degree. C.) or a cold air (15.degree. C.) supplied by an independent fan. Now, under fluidization, the air flow required is proportional to the area of the bed to be fluidized. It can be seen in this case that the flow of the air to be heated is twice as large as the flow which would be useful if the air passed from one stage to another. Furthermore, during cooling in the two stages provided for this purpose, the heat yielded up by the maize is not recovered, it is partly for these two reasons that the energy consumption is fairly high (of the order of 1400 kg/cal per kilo of water evaporated). PA1 the heat supplied for heating the grain is lost, PA1 the internal diffusion of the water is slowed down, PA1 energy is used for fluidizing. PA1 an external cylindrical casing with vertical axis in which is mounted at least one stationary horizontal circular plate forming the lower limit of a fluidized bed stage, a shaft capable of pivoting on itself being mounted along this axis, said shaft having as many series of radial blades as there are plates, said blades, which define compartments for the material to be dried displacing the material present on said plate, each plate having a perforated part corresponding to the drying fluid passage zone, a closed part corresponding to a sweating zone and an open part, the material to be dried arriving on a plate on a section of its closed part and then passing onto a perforated part, then onto a closed section for sweating, or alternately over several perforated and closed parts and being at the end of travel, discharged by its open part; PA1 a means for causing the rotational movement of the shaft; and PA1 a means for transporting the treatment fluid in the upward direction. PA1 a parallelepipedic casing with vertical axis in which is mounted at least one perforated horizontal plate forming the lower limit of a fluidized bed stage, a blade being capable of moving perpendicularly to said plate from one end to the other thereof while closing or opening, depending on whether it is in one or other of its end positions, an aperture formed in the wall of the casing above said plate, two external lateral hoppers being associated with each plate, each hopper being defined by an external wall common to the whole of the apparatus and by a bottom located in the same plane as said plate and capable of occupying a position adjacent thereto for filling the hopper and sweating of its contents or an externally offset position for discharging the contents of the hopper to the lower stage, a blade perpendicular to the bottom of each hopper being mounted for translational movement between said external wall and the wall of the casing comprising said aperture and being capable of closing this aperture in its corresponding end position; PA1 a means for moving the blades and the bottoms of the hoppers in translation; and PA1 a means for transporting the treatment fluid in the upward direction in a single flow.
The limiting step in drying the maize is the diffusion of water from inside the grain towards the surface, this diffusion is improved by heating the grain and it is advantageous to maintain it at a given temperature, for a given time, without any circulation of drying air, so as to accelerate the diffusion without an additional supply of energy.
In the Bulgarian drier, the fact of cooling the grain between two drying steps is an energy consuming action, since:
Existing driers have never succeeded in providing simultaneously the uniformity of the residence times of the grain and efficient fluid flow from the energy point of view. The present invention allows to conciliate these objects by providing a compartmentalized flow of the grain and passage of the drying air from one drying stage to another in a countercurrent wise with respect to the material to be dried.
The uniformity of the residence times of the grain in the different sections of the apparatus is important for conciliating the thorough drying of the grain and the maintainance of its food qualities. The migration of humidity in the grain requires a certain time for diffusion, this diffusion being faster if the temperature of the grain increases. On the other hand, the loss of the food qualities (or others) depends essentially on the time during which it is maintained at a high temperature. In order to provide at best required compromise between drying speed and maintenance of quality, the temperature and drying time should be chose optimally. This is only possible if all the grains have the same thermal history. To the extent that the residence times are not the same for all the grains in a given stage, assumed as being at an homogeneous temperature, it is not possible to provide the best compromise for all the grains. Some grains held at a high temperature for a short period of time will not be dry, whereas others kept at a high temperature for too long a time will have lost their qualities (they will be cooked).
Attempts have therefore been made to provide a favourable circulation of the grains and of the drying fluid, making possible an homogenous distribution of the residence times, which leads to partitioning the fluid bed of grains which, without that, would be totally mixed and would lead to a wide spread of the residence times in the "population" of the grains. The partitioning itself must be movable, so as to make possible the displacement of the grains in a continuous process, that is to say where the temperature remains constant as a function of time at a given point.